Excimer lamps are quasimonochromatic light sources operating over a wide range of wavelengths in the ultraviolet (UV) and vacuum ultraviolet (VUV) spectral regions. Operation of an excimer lamp is based on the formation of excited dimers (excimers), which spontaneously transiting from the excited state to the ground state result in the emission of UV-photons. The spectral maximum of excimer lamp radiation is specified by a working excimer molecule (see table below).
Wavelength and photon energy of excimer lamp radiation.
Excimers are diatomic molecules (dimers) or polyatomic molecules that have stable excited electronic states and an unbound or weakly bound (thermally unstable) ground state. Initially, only homonuclear diatomic molecules with a stable excited state but a repulsive ground state were called excimers (excited dimers). The term "excimer" was later extended to refer any polyatomic molecule with a repulsive or weakly bound ground state. One can also come across the term "exciplex", meaning an excited complex. It is also an excimer molecule but not a homonuclear dimer. For instance, Xe2*, Kr2*, Ar2* are excimer molecules, while XeCl*, KrCl*, XeBr*, ArCl*, Xe2Cl* are referred to exciplex molecules. Dimers of rare gases and rare gas-halogen dimers are the most spread and studied excimers. Rare gas-halide trimers, metal excimers, metal-rare gas excimers, metal-halide excimers, and rare gas-oxide excimers are also known,[4] but they are rarely used.
An excimer molecule can exist in an excited electronic state for a limited time, as a rule from a few to a few tens of nanoseconds. After that, an excimer molecule transits to the ground electronic state, while releasing the energy of internal electronic excitation in the form of a photon. Owing to a specific electronic structure of an excimer molecule, the energy gap between the lowest bound excited electronic state and the ground state amounts from 3.5 to 10eV, depending on a kind of an excimer molecule and provides light emission in the UV and VUV spectral region. A typical spectral characteristic of excimer lamp radiation consists mainly of one intense narrow emission band.[5] About 70-80% of the whole radiation power of an excimer lamp is concentrated in this emission band. The full-width at half maximum of the emission band depends on a kind of an excimer molecule and excitation conditions and ranges within 2 to 15nm. In fact, excimer lamps are sources of quasimonochromatic light. Therefore, such sources are suitable for spectral-selective irradiation and can even replace lasers in some cases.[6][7][8]
UV production
Radiation is produced owing to the spontaneous transition of an excimer molecule from an excited electronic state to the ground state. Excimer and exciplex molecules are not long-living formations. They rapidly decompose typically within a few nanoseconds, releasing their excitation energy in the form of a UV photon:
emission of an excimer molecule:
emission of an exciplex molecule:
where Rg2* is an excimer molecule, RgX* is an exciplex molecule, Rg is an atom of rare gas, and X is an atom of halogen.
Excimer molecule formation
It is convenient to generate excimer molecules in a plasma. Electrons play an important role in a plasma and, in particular, in the formation of excimer molecules. To be able generating efficiently excimer molecules, the working medium (plasma) should contain sufficient concentration of electrons with energies that are high enough to produce the precursors of the excimer molecules, which are mainly excited and ionized rare gas atoms. Introduction of power into a gaseous mixture results in the formation of excited and ionized rare gas atoms as follows:
where Rg* is a rare gas atom in an excited electronic state, Rg+ is a rare gas ion, and e− is an electron.
When there are enough excited rare gas atoms accumulated in a plasma, the excimer molecules are formed by the following reaction:
Rg* + Rg + M → Rg2* + M,
where Rg2* is an excimer molecule, and M is a third particle carrying away the excess energy to stabilize an excimer molecule. As a rule, it is a rare gas atom of the working medium.
Analyzing this three-body reaction, one can see that the efficiency of the production of excimer molecules is proportional to the concentration of excited rare gas atoms and the square of the concentration of rare gas atoms in the ground state. From this point of view, the concentration of rare gas in the working medium should be as high as possible. A higher concentration of rare gas is achieved by increasing gas pressure. However, an increase in the concentration of rare gas also intensifies the collisional quenching of excimer molecules, resulting in their radiationless decay:
Rg2* + Rg → Rg* + 2Rg.
The collisional quenching of excimer molecules is negligible while the mean time between collisions is much higher than the lifetime of an excimer molecule in an excited electronic state. In practice, the optimal pressure of a working medium is found experimentally, and it amounts approximately to one atmosphere.
A mechanism underlying the formation of exciplex molecules (rare gas halides) is a bit more complicated than the mechanism of excimer molecule formation. The formation of exciplex molecules occurs in two main ways. The first way is due to a reaction of ion-ion recombination, i.e., recombination of a positive rare gas ion and a negative halogen ion:
Rg+ + X− + M → RgX* + M,
where RgX* is an exciplex molecule, and M is a collisional third partner, which is usually an atom or molecule of a gaseous mixture or buffer gas. The third particle takes the excess energy and stabilizes an exciplex molecule.
The formation of a negative halogen ion results from the interaction of a low-energy electron with a halogen molecule in a so-called process of the dissociative electron attachment:
X2 + e− → X + X−,
where X is a halogen atom.
The pressure of a gaseous mixture is of great importance for efficient production of exciplex molecules due to the reaction of ion-ion recombination. The fact is that the reaction of ion-ion recombination is a process of a three-body collision, and the probability of a triple collision increases with pressure. At low pressures of a gaseous mixture (several tens of torrs), the reaction of ion-ion recombination is of little efficiency, while it is quite productive at pressures above 100 Torr.
The second way of the formation of exciplex molecules is a harpoon reaction. In this case, a halogen molecule or halogen-containing compound captures a weakly bound electron of an excited rare gas atom, and an exciplex molecule in an excited electronic state is formed:
Rg* + X2 → RgX* + X.
Since the harpoon reaction is a process of a two-body collision, it can proceed productively at a pressure significantly lower than that required for a three-body reaction. Thus, the harpoon reaction makes possible the efficient operation of an excimer lamp at low pressures of a gaseous mixture. The collisional quenching of exciplex molecules at low pressures of a gaseous mixture is much lower than at pressures required for productive proceeding the reaction of ion-ion recombination. Due to this, a low-pressure excimer lamp ensures the maximum efficiency in converting the pumping energy to UV radiation.
It should be mentioned that both the harpoon reaction and reaction of ion-ion recombination proceed simultaneously. The dominance of the first or second reaction is mainly determined by the pressure of a gaseous mixture. The harpoon reaction predominates at low pressures (below 50 Torr), while the reaction of ion-ion recombination prevails at higher pressures (above 100 Torr).
The kinetics of reactions proceeding in a plasma is diverse and is not limited to the processes considered above. The efficiency of producing exciplex molecules depends on the composition of a gaseous mixture and conditions of its excitation. Type of a halogen donor plays an important role. The most effective and widely used halogen-carriers are homonuclear diatomic halogen molecules. More complex halogen compounds such as hydrogen halides, metal halides, and interhalogens are also used as a halogen-carrier but to a lesser extent.
A noteworthy halogen-carrier is alkali halide. A feature of alkali halides is a similarity of their chemical bond with that of exciplex molecules in excited electronic states. Exciplex molecules in excited electronic states are characterized by the ionic bond as well as alkali halides in the ground state. It opens up alternative mechanisms for the formation of exciplex molecules, namely substitution reactions:
Rg* + AX → RgX* + A,
Rg+ + AX → RgX* + A+,
where AX is an alkali halide molecule, A is an alkali metal atom, and A+ is an alkali metal ion.
These mechanisms of the formation of exciplex molecules are fundamentally different from the reaction of ion-ion recombination and harpoon reaction.[9] An exciplex molecule is formed simply by replacing an atom/ion of alkali metal from an alkali halide molecule by an excited atom/ion of rare gas.
An advantage of using alkali halides is that both the substitution reactions can simultaneously proceed at low pressures with comparable productivity.[10] Moreover, both excited atoms and ions of rare gas are effectively used in the production of exciplex molecules in contrast to excimer lamps using other halogen-carriers. It is of importance because the ionization and excitation of rare gas consume most of the introduced energy. Since the reaction of ion-ion recombination and harpoon reaction dominate depending on the pressure of a gaseous mixture, the generation of rare gas ions is unprofitable at low pressures, while the excitation of rare gas is unreasonable at high pressures. A drawback of using alkali halides is high temperatures required for providing the necessary concentration of alkali halide molecules in a gaseous mixture. Despite this, the use of alkali halides as a halogen-carrier is especially promising in the development of exciplex lasers operating at low pressures.[10]
Methods of excitation
One of the widely used ways to excite emission of excimer molecules is an electric discharge. There are a lot of discharge types used for pumping excimer lamps. Some examples are glow discharge, pulsed discharge, capacitive discharge, longitudinal and transverse discharges, volume discharge, spark discharge, and microhollow discharge. As of 2013[update], dielectric barrier discharge (DBD), a type of capacitive discharge, is the most common type used in commercial lamps.[11][12] A benefit of the DBD excimer lamps is that the electrodes are not in direct contact with the active medium (plasma). Absence of interaction between the electrodes and the discharge eliminates electrode corrosion as well as contamination of the active medium by sputtered electrode material, which considerably increases the lifetime of DBD excimer lamps in comparison with others. Moreover, a dielectric barrier discharge ensures effective excitation of a gas mixture in a wide range of working pressures from a few torrs to more than one atmosphere. Excimer lamps can be made in any desired shape of the radiating surface, satisfying requirements of a specific task.
Benefits of excimer lamps
The main advantages of excimer lamps over other sources of UV and VUV radiation are as follows:
high average specific power of UV radiation (up to 1 Watt per cubic centimeter of active medium);
high energy of an emitted photon (from 3.5 to 11.5eV);
quasimonochromatic radiation with the spectral full-width at half maximum from 2 to 15nm;
high power spectral density of UV radiation;
choice of the wavelength of the spectral maximum of UV radiation for specific purposes (see table);
availability of multi-wave UV radiation owing to simultaneous excitation of several kinds of working excimer molecules;
absence of visible and IR radiation;
instant achievement of the operating mode;
low heating of radiating surface;
absence of mercury.
Applications
Commercially-used 172 nm excimer lamp for printing industry
Light sources emitting in the UV spectral region are widely used in techniques involving photo-chemical processes, e.g., curing of inks, adhesives, varnishes and coatings, photolithography, UV induced growth of dielectrics,[13] UV induced surface modification, and cleaning or material deposition. Incoherent sources of UV radiation have some advantages over laser sources because of their lower cost, a huge area of irradiation, and ease of use, especially when large-scale industrial processes are envisaged.
Mercury lamps (λ = 253.7nm) are widely spread UV sources, but their production, use, and disposal of old lamps pose a threat to human health and environmental pollution. Comparing with commonly used mercury lamps, excimer lamps have a number of advantages. A specific feature of an excimer molecule is the absence of a strong bond in the ground electronic state. Thanks to this, high-intensity UV radiation can be extracted from a plasma without significant self-absorption. This makes possible to convert efficiently energy deposited to the active medium into UV radiation.
Excimer lamps are referred to cold sources of UV radiation since the radiating surface of excimer lamps remains at relatively low temperatures in contrast with traditional UV lamps like a mercury one. Because the medium does not need to be heated, excimer lamps reach their peak output almost immediately after they are turned on.
Rare gas and rare gas-halide excimer lamps generally radiate in the ultraviolet (UV) and vacuum-ultraviolet (VUV) spectral regions (see table). Their unique narrow-band emission characteristics, high quantum efficiency, and high-energy photons make them suitable for applications such as absorption spectroscopy, UV curing, UV coating, disinfection, ozone generation, destruction of gaseous organic waste, photo-etching and photo-deposition and more other applications.[14]
Light sources emitting photons in the energy range of 3.5–10eV find applications in many fields due to the ability of high-energy photons to cleave most chemical bonds and kill microbes destroying nucleic acids and disrupting their DNA. Examples of excimer lamp applications include purification and disinfection of drinking water, pool water, air, sewage purification, decontamination of industrial waste, photochemical synthesis and degradation of organic compounds in flue gases and water, photopolymerization of organic coatings and paints, and photo-enhanced chemical vapor deposition.[15][16] In all cases UV photons excite species or cleave chemical bonds, resulting in the formation of radicals or other chemical reagents, which initiate a required reaction.
An excimer lamp has selective action. UV radiation of a given wavelength can selectively excite species or generate required radicals. Such lamps can be useful for photophysical and photochemical processing such as UV curing of paints, varnishes, and adhesives, cleansing and modifying surface properties, polymerization of lacquers and paints, and photo-degradation of a variety of pollutants. Photo-etching of polymers is possible using different wavelengths: 172nm by xenon excimer, 222nm by krypton chloride, and 308nm by xenon chloride. Excimer UV sources can be used for microstructuring large-area polymer surfaces. XeCl-excimer lamps (308nm) are especially suitable to get tan.
Fluorescence spectroscopy is one of the most common methods for detecting biomolecules. Biomolecules can be labeled with fluoroprobe, which then is excited by a short pulse of UV light, leading to re-emission in the visible spectral region. Detecting this re-emitted light, one can judge the density of labeled molecules. Lanthanide complexes are commonly used as fluoroprobes. Due to their long lifetime, they play an important role in Forster resonance energy transfer (FRET) analysis.
At present, excimer lamps are coming into use in ecology, photochemistry, photobiology, medicine, criminalistics, petrochemistry, physics, microelectronics, different engineering tasks, wide-ranging technologies, science, various branches of industry including the food industry, and many others.
Environmental contamination
Mercury lamps are the most common source of UV radiation due to their high efficiency. However, the use of mercury in these lamps poses disposal and environmental problems. On the contrary, excimer lamps based on rare gases are absolutely non-hazardous and excimer lamps containing halogen are more environmentally benign than mercury ones.[citation needed]
Related Research Articles
Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) is a spectroanalytical procedure for the quantitative determination of chemical elemlight) by free atoms in the gaseous state. Atomic absorption spectroscopy is based on absorption of light by free metallic ions.
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.
The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a practical tube that could detect a number of different radiation types.
In chemistry, a halide is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, astatide, or theoretically tennesside compound. The alkali metals combine directly with halogens under appropriate conditions forming halides of the general formula, MX. Many salts are halides; the hal- syllable in halide and halite reflects this correlation. All Group 1 metals form halides that are white solids at room temperature.
An excimer is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has a valence shell completely filled with electrons. In this case, formation of molecules is possible only if such atom is in an electronic excited state. Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules. Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds.
An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micromachining. Since 1960s excimer lasers are widely used in high-resolution photolithography machines, one of the critical technologies required for microelectronic chip manufacturing.
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate on the principle of converting electrical energy to a laser light output. The first gas laser, the Helium–neon laser (HeNe), was co-invented by Iranian-American engineer and scientist Ali Javan and American physicist William R. Bennett, Jr., in 1960. It produced a coherent light beam in the infrared region of the spectrum at 1.15 micrometres.
A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides. It is a type of high-intensity discharge (HID) gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light. The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes. As a result, metal-halide lamps have high luminous efficacy of around 75–100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights and produce an intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public places, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting, automotive headlamps and indoor cannabis grow operations.
The induction lamp, electrodeless lamp, or electrodeless induction lamp is a gas-discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside. This is in contrast to a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. Eliminating the internal electrodes provides two advantages:
A krypton fluoride laser is a particular type of excimer laser, which is sometimes called an exciplex laser. With its 248 nanometer wavelength, it is a deep ultraviolet laser which is commonly used in the production of semiconductor integrated circuits, industrial micromachining, and scientific research. The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically contains a mixture of: a noble gas such as argon, krypton, or xenon; and a halogen gas such as fluorine or chlorine. Under suitably intense conditions of electromagnetic stimulation and pressure, the mixture emits a beam of coherent stimulated radiation as laser light in the ultraviolet range.
A deuterium arc lamp is a low-pressure gas-discharge light source often used in spectroscopy when a continuous spectrum in the ultraviolet region is needed.
Dielectric-barrier discharge (DBD) is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent (inaudible) discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. With rare exceptions, krypton is chemically inert.
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Xenon monochloride (XeCl) is an exciplex which is used in excimer lasers and excimer lamps emitting near ultraviolet light at 308 nm. It is most commonly used in medicine. Xenon monochloride was first synthesized in the 1960s. Its kinetic scheme is very complex and its state changes occur on a nanosecond timescale. In the gaseous state, at least two kinds of xenon monochloride are known: XeCl and Xe 2Cl, whereas complex aggregates form in the solid state in noble gas matrices. The excited state of xenon resembles halogens and it reacts with them to form excited molecular compounds.
The helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms. This chemical is the largest diatomic molecule—a molecule consisting of two atoms bonded together. The bond that holds this dimer together is so weak that it will break if the molecule rotates, or vibrates too much. It can only exist at very low cryogenic temperatures.
Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.
Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the CSIRO.
↑ Saburoh Satoh; Takao Tanaka; Satoshi Ihara; Chobei Yamabe (2000). Chen, Xiangli; Fujioka, Tomoo; Matsunawa, Akira (eds.). "Stereolithography with XeCl excimer laser/lamp". Proceedings of the SPIE. High-Power Lasers in Manufacturing. 3888: 264–271. Bibcode:2000SPIE.3888..264S. doi:10.1117/12.377028.
↑ Saburoh Satoh; Takao Tanaka; Satoshi Ihara; Chobei Yamabe (2000). Helvajian, Henry; Sugioka, Koji; Gower, Malcolm C; etal. (eds.). "Excimer lamp stereolithography". Proceedings of the SPIE. Laser Applications in Microelectronic and Optoelectronic Manufacturing V. 3933: 272–279. Bibcode:2000SPIE.3933..272S. doi:10.1117/12.387563.
↑ K. Köllner; M.B. Wimmershoff; C. Hintz; M. Landthaler; U. Hohenleutner (2005). "Comparison of the 308-nm excimer laser and a 308-nm excimer lamp with 311-nm narrowband ultraviolet B in the treatment of psoriasis". British Journal of Dermatology. 152 (4): 750–754. doi:10.1111/j.1365-2133.2005.06533.x. PMID15840108.
↑ M.S. Klenovskii; V.A. Kel’man; Yu.V. Zhmenyak; Yu.O. Shpenik (2013). "Luminescence of XeCl* and XeBr* exciplex molecules initiated by a longitudinal pulsed discharge in a three-component mixture of Xe with CsCl and CsBr vapors". Optics and Spectroscopy. 114 (2): 197–204. Bibcode:2013OptSp.114..197K. doi:10.1134/S0030400X13010141.
1 2 A.M. Boichenko; M.S. Klenovskii (2015). "Laser generation of XeCl exciplex molecules in a longitudinal repetitively pulsed discharge in a Xe – CsCl mixture". Quantum Electronics. 45 (12): 1105–1110. Bibcode:2015QuEle..45.1105B. doi:10.1070/QE2015v045n12ABEH015859.
↑ U. Konelschatz; B. Eliasson; W. Egl (1997). "Dielectric-Barrier Discharges. Principle and Applications". J. Phys. (Paris). Series IV. 7 (C4): 47–66. doi:10.1051/jp4:1997405.
↑ Ulrich Kogelschatz (2003). "Dielectric-Barrier Discharges: Their History, Discharge Physics, and Industrial Applications". Plasma Chemistry and Plasma Processing. 23 (1): 1–46. doi:10.1023/A:1022470901385.
↑ Edward A. Sosnin; Thomas Oppenländer; Victor F. Tarasenko (2006). "Applications of capacitive and barrier discharge excilamps in photoscience". Journal of Photochemistry and Photobiology C: Photochemistry Reviews. 7 (4): 145–163. doi:10.1016/j.jphotochemrev.2006.12.002.
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